MAXI Observations of Supergiant Fast X-ray Transients
Ai Sakamaki and Hitoshi Negoro
Department of Physics, Nihon University
1-8 Kanda-Surugadai, Chiyoda-ku, Tokyo, Japan
E-mail(AS): sakamaki@cygnus.phys.cst.nihon-u.ac.jp
Abstract
About 20 supergiant fast X-ray transients (SFXTs) have been discovered mainly by INTEGRAL. The
MAXI nova-alert system was often triggered by short X-ray flares or outbursts probably from SFXTs,
for instance, IGR J18483−0311, AX J1841.0−0536, and AX 1739.1−3020 (aka XTE J1739−302/IGRJ17391−3021) even though about half of SFXTs can not be spatially resolved from nearby bright X-raysources. Thus, MAXI may discover new SFXTs in future (a present SFXT candidate is MAXI J1932+091).
Here, we briefly summarize preliminary MAXI results of SFXTs, and present the capability to detect theorbital periodicity of IGR J18483−0311.
Key words: X-rays: transient — neutron star: SFXTs

1. Introduction
Supergaint Fast X-ray Transients, SFXTs, (e.g.,Sgueraet al. 2005; Negueruela et al. 2006) are a sub-class ofhigh mass X-ray binaries (HMXBs). SFXTs exhibit relatively short, and not-so-bright hard X-ray flares. Theflare activities typically last less than one hour, and thepeak flux of the flares are around 100 mCrab or less inthe soft X-ray band, which are just below the detection
limits of the previous all sky monitors, e.g., Gigna/ASM
and RXTE/ASM. These characteristics of the sources
had long hidden their nature until INTEGRAL discovered them.
The MAXI nova-alert system (Negoro et al. 2016) was
often triggered by probable short transient activities ofSFXTs in a single scan transit or 4 orbits (∼ 6 h) timebin. If detected, we, the MAXI team, provide a prompt
E-mail alert to the members of the x-ray-star MAXI
mailing list. Here, we briefly summarize observational
properties of SFXTs from MAXI/GSC observations.
2. MAXI detections of SFXTs
About half of SFXTs are so close to bright sources thatMAXI can not resolve the sources. Such sources areshown in light green in figure 1 (see a color version on thewebsite). Furthermore, X-ray properties of flares fromSFXTs, ”short and hard”, make it diﬃcult for MAXI tocatch flares. Nevertheless, X-ray enhancements probably due to activities of SFXTs often triggered the MAXI
nova-alert system, or were recognized on GSC images,e.g., AX J1841.0−0536 (Negoro et al. 2010) and XTE
J1739−302/IGR 17391−3021 (Negoro et al. 2015). Usu-ally, such enhancements were recognized only in a single
scan orbit, except for outbursts from IGR J18483−0311.
We, however, could not confirm that the activities werereally due to SFXTs except for simultaneous detection
with other observatories.
Such possible detections of SFXTs with MAXI/GSCare summarize in table 1. As shown in the table, thenumbers of the MAXI detections are positively correlated with those of Swift/BAT detections, suggestingthat MAXI actually detected the source activities. We
are checking the significance of each detection and thesource location more precisely by taking point spreadfunctions of the cameras and nearby sources into account.
3. Periodicity
Periodicity search was performed for IGR J18483−0311
using GSC data over 7 years, and the orbital periodof ∼ 18.5 days was clearly detected in the power spectrum density (figure 2). Sakakibara (2013) obtained amore precise period of 18.576 ± 0.016 days using 4 years
MAXI/GSC data.
He also showed that a 2–20 keV folded light curveof GSC data had a profile very similar to a 15–50 keVlight curve of Swift/BAT data, and that the soft X-rays
tended to increase more gradually than the hard X-rays.
Such gradual increase in the soft X-ray band was also
recognized in RXTE/ASM data (Levine, et al. 2011).
4. Discussion and Future Works
As shown above, MAXI can detect SFXT activities.
This implies that undiscovered SFXTs will be detectedGX 3+1
IGR J18462-0223

IGRIG
R J1J1747407
07-2808

IGR J16418-4532
AX J1739
39.1
39.1-3.1-3020-34U 1630-472

AX J1749.1-27333

GX 13+12XMM
2X
MM J185114.3-000004
IGR J19294+1816

AX J1841.0-053
536536SAX J1818.6-1703

XTE J1901+01014
014

IGR J1121215-5952

IGRIG
R J1626207-5-512-5129
12IGR
R J1
J1646465
6465-465-450-45077
50IGR
R J0J08408
08-4-4503

IGR J1J1646479
6479-479-451-45144
51IGR
IG
R J1
J17575447544-244-261
6199
61

IGR
IG
R J1
J161
6195
95-4949455

Cen X-3

IGR J1
J173
7354
7354-354-325255
MAXI J1932+091

IGR J18483
83-0-031-0311
IGR
IG
R J1
J181
8175-281-241-24199
41
AX J1845.0-0433

Vela Pulsar

IGR
R J1
J163
6328-472
7266
72

GX 340+0

Fig. 1. Locations of known SFXTs and nearby bright sources shown on a MAXI GSC+SSC X-ray image.

Table 1. List of SFXTs spatially resolved by MAXI (shown in yellow in figure 1)(264.041, −1.950)(332.459, −1.050)

MAXI
Detection142?

BAT
Detection1
10(358.068, +0.445)

8?

27(26.764, −0.239)

1026(28.138, −0.658)

3?

(29.750, −0.745)(32.909, −0.043)(35.381, −1.623)(53.540, +0.115)

Object Name(l, b)IGR J08408−4503
IGR J16207−5129
AX J1739.1−3020/XTE J1739−302/IGR J17391−3021
AX J1841.0−0536
AX J1845.0−0433/
IGR J18450−0435
IGR J18483−0311(+ IGR J18462−0223)
2XMM J185114.3
XTE J1901+014
IGR J19294+1816

Nearby Source
Namef 2 [mCrab]
Vela Pulsar∼ 10∆θ[d]0.54

IGR J1735−3022
XB 1732−304∼200.350.74

14

Swift J1843.5−0343∼ 100.863557

GS 1843−02∼ 100.75(0.38)

032?

21. All the numbers of the MAXI detections are preliminary. Those of the BAT detections are from Romano et al. (2014) exceptfor XTE J1901+014 (Krimm et al. 2010). The numbers show total days, not the times of triggers or outbursts.
2. Steady 4-10 keV X-ray flux, including the galactic ridge X-ray emission, observed with GSC at the nearby source region.

1.5×10−310−3
5×10−40intensity [arbitrary]

2×10−3

not be excluded.
On the other hand, we have to carefully reanalyze
MAXI data of known SFXTs as described above to evaluate long-term source activities or its recurrence in thesoft X-ray band, which have not been extensively investigated after e.g., Levine, et al. (2011).

02×10−7

4×10−7
6×10−7frequency [Hz]8×10−710−6Fig. 2. PSD of IGR J18483−0311 (MJD55058-57692, 4.0-10.0 keV)in future. In fact, MAXI J1932+091 (Yamaoka et al.
2014; Kennea et al. 2014), of which the companion is
likely to be a Be star (Itoh et al. 2014), is a SFXT candidate though it locates at relatively high galactic latitude,b ∼ −5 deg (see figure 1), and its emission lasted morethan 9 hours (also see Negoro et al. 2016). Of course,
other possibilities, for instance, a black hole binary, canReferences
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